In the hydrodesulfurization of petroleum residua, catalyst cost factors constitute a major problem. These cost factors are an aggregate of catalyst raw material and manufacturing costs, and the activity and deactivation rates of the catalysts. The problem is further aggravated by the fact that to date it has not been found commercially feasible to regenerate deactivated residua desulfurization catalysts, due principally to the deposition thereon during processing of metals such as vanadium and nickel, which are universally present in residual feedstocks.
Balancing all of the foregoing factors, the most cost-effective type of catalyst yet discovered for residual oil desulfurization is composed of minor proportions of cobalt and molybdenum dispersed in an alumina support. It appears that the most inexpensive and widely used method for manufacturing such catalysts (disclosed for example in U.S. Pat. Nos. 3,509,044 and 3,674,680) involves simply comulling the alumina support, generally a spray-dried alumina hydrate, with an aqueous solution or solutions of cobalt and molybdenum compounds, extruding the mixture and calcining the extrudates. Though relatively inexpensive, this method generally produces catalysts of low intrinsic activity and/or high deactivation rates, such that very large quantities of catalyst are required for a given feed throughput and conversion, and the catalyst deactivation rate is often such that only short run lengths are obtainable before the catalyst must be discarded and replaced.
I have now discovered that by resorting to a slightly more expensive manufacturing method, a catalyst of such improved activity and activity maintenance is obtained as to render it overall more cost-effective than the above discussed prior art catalysts. In brief summary this method involves the following steps:
1. SLURRYING AN ALUMINA HYDROGEL, WITH OR WITHOUT A MINOR PROPORTION OF SILICA HYDROGEL, IN AN AQUEOUS SOLUTION OF AMMONIUM MOLYBDATE;
2. SPRAY DRYING THE RESULTING SLURRY TO A WATER CONTENT OF ABOUT 10-40 WEIGHT-PERCENT;
3. MULLING THE SPRAY-DRIED COMPOSITE WITH WATER AND SUFFICIENT OF A PEPTIZING ACID TO GIVE A PH between about 3.6 and 6.0;
4. extruding the mulled composite into extrudates having a maximum overall diameter between about 0.03 and 0.06 inches, and a length between about 0.1 and 0.25 inches;
5. calcining the resulting extrudates at a temperature between about 900.degree. and 1400.degree.F;
6. impregnating the calcined extrudates with an aqueous solution of a cobalt compound; and
7. calcining the impregnated extrudates at a temperature between about 900.degree. and 1400.degree.F.
Catalysts thus prepared have been found to display in general about 30 to 70 percent higher activity than catalysts of the same nominal composition prepared by the above discussed prior art method. While I am unable to account with certainty for this improved activity, it is hypothesized that my method permits a more complete and selective chemical and/or physico-chemical combination of the molybdenum component with the alumina base, rather than with the cobalt component. The above described prior art method inherently results in an uncontrolled and indiscriminate interaction of all components with each other during the single calcination step. Further, by suitably controlling the severity of calcination in step (5), I am able to obtain a product of high surface area such that upon subsequent impregnation with the cobalt solution in step (6) a maximum cobalt surface area is obtained. The final product is also found to display an optimum combination of the critical physical characteristics of total surface area, pore volume, and pore size distribution for the desulfurization of residua feedstocks. The catalysts may be utilized to obtain any desired degree, up to about 95 percent, of desulfurization of most conventional residua feedstocks. Run lengths in the order of about 6 months to 1 year are normally obtainable.